Comprehensive Income (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2016	Sep. 30, 2015	Sep. 30, 2016	Sep. 30, 2015
Comprehensive Income Detail [Abstract]
Comprehensive income available for common stockholders	$ 20	$ 14	$ 83	$ 27
Earnings per share on comprehensive income:
Basic	$ 0.31	$ 0.23	$ 1.27	$ 0.45
Diluted	$ 0.30	$ 0.23	$ 1.25	$ 0.44
Weighted average common shares outstanding:
Weighted Average Number of Shares Outstanding, Basic	67,083,654	62,458,655	65,351,842	60,152,525
Weighted Average Number of Shares Outstanding, Diluted	68,470,224	64,028,731	66,738,754	61,646,928

Amount after tax of increase (decrease) in equity from transactions and other events and circumstances from net income and other comprehensive income, attributable to parent entity. Excludes changes in equity resulting from investments by owners and distributions to owners.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Other Comprehensive Income
-URI http://asc.fasb.org/extlink&oid=51831270

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Comprehensive Income
-URI http://asc.fasb.org/extlink&oid=51831223

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=36458714&loc=d3e557-108580

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Glossary Net Income
-URI http://asc.fasb.org/extlink&oid=51831255

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